TITLE: GCN CIRCULAR NUMBER: 11766 SUBJECT: IBAS distribution of coordinates for low significance INTEGRAL GRB triggers DATE: 11/02/24 16:18:42 GMT FROM: Sandro Mereghetti at IASF/CNR S.Mereghetti (INAF IASF-Milano), D.Gotz (CEA-Saclay), J. Borkowski (CAMK, Torun), M. Beck, C.Ferrigno (ISDC, Versoix), on behalf of the IBAS Localization Team announce: A new version of the IBAS Alert Distribution system is operational at the INTEGRAL Science Data Center. In addition to the already available functionalities, the new version distributes in real time also Alert Packets for low significance triggers discovered in the field of view of the IBIS imager. The new Alert Packets are called WEAK Alerts and are identified by the field PKT_TYPE=6 (see http://www.isdc.unige.ch/integral/science/grb#IBAS for details on the different Alert types). Users interested to receive the Alert Packets of the new type "WEAK" should contact the ISDC IBAS team (isdc-ibas@unige.ch). The threshold for the distribution of the usual high significance IBAS triggers remains unchanged at SIGMA= 8.0 (Alert Packets with PKT_TYPE= 3 (WAKEUP)) while the threshold for the distribution of low significance IBAS triggers is currently set at SIGMA= 6.5 (Alert Packets with PKT_TYPE= 6 (WEAK)) This threshold corresponds to a rate of a few WEAK Alert Packets per week, but it could be adjusted in the future. Many WEAK alerts will be produced by statistical fluctuations, but there is also a non negligible probability that some of them are due to real astrophysical events. In most cases it is not possible to recognize the GRBs based only on the INTEGRAL data, but the detection of afterglows could confirm some of these events as genuine GRBs. Several bursts that were confirmed by off-line interactive analysis of the INTEGRAL data produced triggers above the threshold currently adopted for the WEAK alerts. Recent examples include the two short bursts GRB 100703A (GCN Circ. n.10936) and GRB 110112B (GCN Circ. n.11562) and the long burst GRB 100909A (GCN Circ. n.11260). WEAK Alert Packets provide a single sky position, with error radius smaller than 4 arcmin. They are intended for robotic telescopes which can react without a negative impact on scheduled observing programmes. The value of SIGMA in the WEAK Alert Packet can be used as a figure of merit. WEAK Alert Packets may or may not be followed by the usual WAKEUP and/or REFINED Alert Packets, depending on the evolution of the detected signal. WEAK alerts for events belows WAKEUP threshold are neither confirmed nor cancelled with an OFFLINE packet. For further details see http://ibas.iasf-milano.inaf.it/IBAS_News.html It is recommended that current ibas clients upgrade their IBAS Client Software to the version 2.1.0 (available at http://www.isdc.unige.ch/integral/analysis#Software), although the previous versions should process WEAK alert without problems.